Method for cooling the cast strand in curved-guide continuous casting plants

ABSTRACT

In the continuous casting of molten steel into a mold having a vertical casting space, from which the cast metal is passed as a molten core surrounded by a peripheral crust on into a curved guide path section, the improvement comprising cooling, while passing through the straight as well as the curved guide path, that side of the metal which is to form the side of greater radius of curvature more intensively than the opposing side to form a crust of greater thickness, while maintaining an intensity of cooling in the region of the straight guide path section approximately 3 to 7 times greater than the intensity of cooling in the curved guide path section, thereby strengthening the cast metal, and thereafter, after deflection of the cast metal into the curved guide path and while the core is still molten, reversing the differential cooling so as to apply a cooling of greater intensity on the side of the lesser radius of curvature, whereby the solidified crusts along all sides of the cast metal assume substantially the same thickness and the cast metal assumes a symmetrical shape at a point prior to complete solidification.

15] 3,656,536 [45] Apr. 18, 1972 FOREIGN PATENTS OR APPLICATIONS ,0665/1954 Germany.................................164/89 ABSTRACT In thecontinuous casting of molten steel into a mold having a vertical castingspace, from which the cast metal is passed as a Primary Exarhiner--R.Spencer Annear Attorney-Imirie and Smiley molten core surrounded by aperipheral crust on into a curved guide path section, the improvementcomprising cooling, while passing through the straight as well as thecurved guide path, that side of the metal which is to form the side ofgreater United States Patent Colombo STRAND IN CURVED-GUIDE CONTINUOUSCASTING PLANTS [72] Inventor: Piero Colombo, 27 Via Leopardi, Udine,

Italy [22] Filed: Nov. 26, 1969 [21] Appl. No.: 880,101

[30] Foreign Application Priority Data Nov. 28, 1968Italy.................;.................7495 A/68 [54] METHOD FORCOOLING THE CAST "u u "n 4 S m "H" 5 mm T n u. g

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- sum 26F 2 J06 581 E S E 408 ,y v4 5 n H65 PRIOR ART INVENToR PiERoCOLOMBO BY 1/ MJIAW BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to a process for the continuous castingof metals, particularly of steel, in which molten metal is continuouslycast in a casting mould having a straight vertical casting space, andpasses as a molten core surrounded by a marginal crust of solidifiedmetal into and through a straight vertical guide path section and asucceeding curved guide path section.

The curved guide path section may be of arcuate form struck about acenter of curvature, or it may be of progressively increasing curvature,for example parabolic or hyperbolic.

2. Description of the Prior Art In the known processes the cast metal,as it passes through the straight vertical path section is cooledequally along sides thereof which are to form the sides of greater andlesser curvature as the cast metal passes through the curved guide pathsection so that the solidified marginal crusts of the cast sectionpossess practically the same thickness along the outer as well as innercurvatures of the cast section.

In all curved continuous casting plants of the said known kind, themarginal crust along the outer curvature of the cast section is stressedin tension in the region of the curved path section, and the marginalcrust along the inner curvature is stressed in compression. Whereas thecompressive stress on the marginal crust along the inner curvature doesnot raise any difficulties as a rule, the tensile stress on the marginalcrust along the outer curvature can lead to the forming of cracks in thetransition area (dendrites area) between the molten core of the castsection and the solidified marginal crust, especially in the case ofsome grades of steel, and even in the solidified marginal crust, if theelongations caused by the tensile strains are too great for the.strength of the material of the cast section.

It is the object of the invention to eliminate these shortcomings and todevelop a process by means of which the deleterious effect of bending ofthe cast section described is eliminated in practice, that is theforming of cracks in the part of the cast section along the outer, thatis the greater curvature, and stresses in tension are prevented or atleast reduced to a degree which is insignificant in respect of thequality of the sections cast.

SUMMARY According to the invention, the problem is resolved by theimprovement which consists of, while the cast metal is passing throughthe straight vertical guide path section, cooling that side of the castmetal which is to form the side of greater curvature while passingthrough the curved guide path section more intensively than the sidewhich is to form the side of lesser curvature thereby to provide theside of greater curvature with a marginal crust of greater thicknessthan that of the marginal crust of the side of lesser curvature.

As a result of the greater thickness of the marginal crust along theside of greater curvature, the zero-line of the distribution of tensileand compressive stress is displaced in the cross-section of the castsection towards the side of greater curvature of the cast section duringbending of the cast section. The maximum specific elongations of thefiber of the material in the part of the cross-section of the castsection corresponding to the greater curvature and proportional to thedistance from this zero-line, are reduced correlatively and cracks whichmay be caused by these elongations in the solidified marginal crustalong the side of greater curvature in the area of the dendrites betweenthe solidified marginal crust and the molten core of the section areprevented.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a vertical section through acurved continuous casting plant for steel,

FIG. 6 is a cross-section through a cast section cooled by the processaccording to the invention, in the region of the line II-II of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT The'curved continuous castingplant for steel illustrated diagrammatically in FIG. 1 comprises awater-cooled continuous casting mold 2 having a straight verticalcasting space which leads into a guide path 3, 4 situated below the mold2. The guide path consists of a straight vertical path section 3situated immediately below and co-axial with the mold 2, and asubsequent arcuate path section 4 that is an arcuate path struck from acenter of curvature. The path sections 3 and 4 are defined by rotatablyarranged guide rollers 5 which are arranged omnilaterally around thecast steel section S in the region of the straight vertical path section3, whereas in the region of the subsequent arcuate guide path section 4,the rollers 5 are situated at least along the outer and inner curvaturesE and A respectively of the cast steel section. Water-spraying norzles6, 106, 206 are arranged to direct water inwards against the cast steelsection S and are situated on all sides around the cast steel section.The noules are located between the guide rollers S in both guide pathsections 3, 4.

The straight vertical guide path section 3 may be of optional height Hbut preferably is between one and two times the height H1 of the mould.The arcuate guide path section 4 may correspond to sector angle C havinga maximum included angle of about The molten steel is cast from ateeming or distributing vessel 7 into the continuous casting mold 2 inwhich a solidified layer 8 of steel, which represents the so-calledmarginal crust of the cast section S, is fonned on all sides along thecooled walls of the mould 2. This solidified marginal crust 8 isinitially in contact with the walls of the mould, that is in the upperportion L1 of the mold 2, and as it slides down the walls of the moldthe thickness of the crust progressively increases. In the lower portionL2 of the mold 2, in which the thickness of the solidified marginalcrust 8 is sufficient to withstand the ferrostatic pressure of themolten core 9 of the cast section, the marginal crust 8 is separatedfrom the walls of the mold due to shrinkage. The cast section S emergingfrom the bottom of the casting mold 2 thus consists of a marginal crust8 solidified all around and of a molten core 9.

After emerging from the mold 2, the cast section S initially traversesthe straight vertical guide path section 3 and is postcooled on allsides by the spraying nozzles 6, 106. The path for the cast section S,defined by the guide rollers 5 of the straight guide path section 3,corresponds to the actual cross-section of the cast section at theegress end of the continuous casting mold 2, within tolerance limits. Inone example, the casting space of the continuous casting mold 2 has asquare cross-section of which each side has a width of 101 mm. 0.4 to0.0). A shrinkage of the cast section is assumed, amounting to 1.3 to1.5 percent of the width of the sides of the casting mold space. Theopen square passage opening for the cast section formed between theguide rollers 5 of the straight guide path section 3 is thus adjustedfor a side-width of 99.5 mm. In the region of transition between thestraight vertical guide path section 3 and the subsequent arcuate guidepath section 4, that is in the region of line 11-1] of FIG. 1, thesection S is deflected, the bending moment being applied by the finalguide roller sets of the guide path section 3 and the first guide rollersets of the guide path section 4. The cast section S, which is thencurved, subsequently traverses the arcuate guide path section and ispost-cooled by the water-spraying noutles 6, 206 until its completesolidification, that is until the molten core 9 of the section hassolidified. The average intensity of the water cooling action on thecast section S, related to the unit of area of the surface of the castsection, is approximately 3 to 7 times and preferably 4 to 6 timesgreater in the region of the straight guide section 3 than the coolingaction effect in the region of the arcuate guide path section 4.Consequently, the thickness of the marginal crust 8 is so increased inthe straight guide path section, that the cast section S can withstandmechanical bending upon entering the arcuate guide path section 4,without incurring damage.

In the known curved continuous casting plants of this kind, the castsection S1, FIG. 5, is cooled with the same intensity along the sidewhich forms the outer curvature E, FIG. 1, as along the side which formsthe inner curvature A of the section at all points along the castsection, below the mould 2. The solidified marginal crust 8 of a castsection Sl obtained by this known cooling method consequently haspractically the same thickness, as indicated diagrammatically in FIG. 5,especially in the terminal portion of the straight vertical guide pathsection 3 along the sides E and A of the cast section. In the region ofbending of this cast section S1, between the straight vertical guidepath section 3 and the subsequent arcuate guide path section 4, thezero-line Nl-Nl separating the tensile portion corresponding to theouter curvature from the compressive portion corresponding to the innercurvature is approximately in the center of the cross-section of thecast section, that is at the same distance 11 from the outer curvature Eof the cast section and from the inner curvature A thereof. In thesecircumstances the elongations engendered by the tensile stresses duringthe bending of the cast section in the portion of the cast sectioncorresponding to the outer curvature can reach so great a value,especially in the case of some grades of steel, that they causeundesirable harmful cracks in the area of transition D (dendrite area)between the molten core 9 of the cast section and the solidifiedmarginal crust 80 along the outer curvature, and even in the marginalcrust 80 along the outer curvature.

According to the invention, each part of the cast section is cooled muchmore intensively along the side thereof which is to form the outercurvature E in the region of the straight vertical guide path section 3than it is along the other sides of the cast section and especially morethan along the side of the cast section which is to form the innercurvature A of the section, that is in such manner that along the sidewhich is to form the outer curvature there is formed a marginal crust108 which has a substantially greater thickness than the marginal crustsalong the other sides of the section and which is especially greaterthan the marginal crust 208 along the side which is to form the innercurvature, as shown in FIGS. 2 and 6. In the region of the bend of thecast section S between the straight vertical guide path section 3 andthe subsequent arcuate guide path section 4, the zero-line N-N, FIG. 6,which separates the tensile portion corresponding to the outer curvaturefrom the compressive portion of the cross-section of the cast sectioncorresponding to the inner curvature is accordingly offset from thecenter of the cast section. The zero-line N--N is displaced considerablytowards the outer curvature E of the cast section as compared with thatof the cast sections S1 according to FIG. which are obtained by theknown cooling methods and are cooled with equal intensity along theouter and inner curvatures of the cast section. As can be seen from FIG.6, the distance hl of the zero-line N-N from the outer curvature E ofthe cast section is considerably less than the distance M of thezero-line NN from the inner curvature A of the cast section.

The maximum specific expansions and compressions of the metal duringbending of the cast section S are directly proportional to the distancehl of the zero-line N-N from the tensionally stressed outer curvature Eto the distance h2 of the zero-line N-N from the compressively stressedinner curvature A of the cast section. Consequently, the maximumspecific elongations in the solidified tensionally stressed portion ofouter curvature of the cast section are so reduced due to the lesserdistance hl from the zero-line N-N, compared to that of the zero-lineN1-N1 of the cast sections S1 according to FIG. 5 which are cooled byconventional methods, that practically no cracks are formed in thesolidified marginal crust 108 along the outer curvature and in thetransition area D (dendritic area) between this marginal crust 108 andthe molten core 9 of the section, and the number of cracks are reducedto an extent which is certainly acceptable qualitatively. At the sametime, the stresses and the maximum specific strains created in thesolidified compressively stressed internal curvature part of the castsection may well be increased according to the greater distance h2 fromthe zero-line N--N as compared to those of the cast sections S1according to FIG. 5, which are cooled by conventional methods, and thehigher tensile and compressive strains can be withstood without damageto the cast section S and do not cause cracks in the marginal crust 208along the internal curvature in the dendritic area between the marginalcrust 208 and the molten core 9 of the cast section. The inner curvatureside A of the cast section and the two lateral sides of the cast sectionare preferably cooled with equal intensity in such manner that themarginal crust 208 along the inner curvature and the two lateralmarginal crusts of the cast section have approximately the samethickness as the corresponding marginal crusts of the cast sections S1obtained by the conventional methods, as can be seen by comparison ofFIGS. 5 and 6.

In addition to the provision of the step of unequal cooling of the outercurvature E and inner curvature A regions of the cast section in theregion of the straight vertical guide path section 3, the positioning ofthe guide path section 3 is of importance. The straight guide pathsection 3 serves not only the purpose of supporting the cast sectionwhich still largely consists of molten steel, but also serves to preventautomatic bending of the cast section S due to the differential coolingaction on the outer and inner curvatures E and A of the section. Thisprevents not only the intervention of harmful strains and deformationsin the solidified marginal crust 8 of the cast section S, but alsoprecludes disturbance in the dendrite formation in the area oftransition between the solidified marginal crust 8 and the molten core 9of the cast section. As a result, the thickness of the solidifiedmarginal crust 8 of the cast section S can increase rapidly and withoutdisturbance during traversal of the straight guide path section 3 untilthe ratio is established which is desirable for the subsequent guidedbending of the cast section S between the thicknesses of the thickermarginal crust 108 along the outer curvature and the thinner marginalcrust 208 along the inner curvature.

The more intensive cooling action on the side E of the cast sectionalong the outer curvature, in the region of the straight guide pathsection 3, may be accomplished in practice by means of optionalstructural devices or measures. For example, it is possible to arrange agreater number of more closely spaced spraying nozzles 106 at the side Eof the section along the outer curvature than are arranged at the othersides of the cast section, as illustrated in FIG. 2. It is also possibleto arrange the sarne number of spraying nozzles at each side of thesection and to employ noules having a higher spraying performance at theside E of the section along the outer curvature. The increase in theintensity of the cooling action on the cast section along the outercurvature in comparison with the cooling action on the other sides ofthe cast section, especially as compared with the cooling action on theinner curvature of the section, depends on the dimensions and the shapeof the cross-section of the section cast, as well as on the propertiesof the steel and on the permissible maximum limits of elongation in thetensionally stressed part of the cross-section of the cast section, forexample, the cooling action on the outer curvature side E of the castsection may be 20 to percent more intensive than the cooling action onany other side of the cast section and particularly on the innercurvature side A of the cast section.

After deflection of the cast section S from the straight vertical guidepath section 3 into the subsequent arcuate guide difference between thehigher intensity of cooling of the outer curvature side E of the castsection and the lower intensity of the cooling of the other sides of thecast section, especially of the inner curvature side A, to be reduced ornullified suddenly or progressively in the longitudinal direction of thecast section, or even reversed, that is in such manner that thesolidified marginal crusts on all sides of the section, and inparticular the marginal crusts 108, 208 along the outer and innercurvature sides E and A of the cast section, assume the same thicknessbefore the complete solidification throughout the cross-section of thecast section. To this end, it is possible to reduce the higher coolingintensity on the outer curvature side A and/or to increase the lowerintensity of the cooling of the other sides of the cast section,especially of the inner curvature side A of the cast section. Thezero-line N-N of the cross-section of the cast section displaced fromthe center in the region on the bend of the cast section S, that isdisplaced in the direction towards the outer curvature side E of thesection, consequently creeps towards the inner curvature side A of thesection and stops in the center of the cross-section of the castsection. The core 9 of the cast section, which is still molten andoffset from the center in the region of the bend accordingly assumes acentral position and the cross-section of the cast section again assumesa symmetrical shape prior to complete solidification of the cast sectionS, as shown in FIG. 4. FIG. 3 shows the cross-section of the castsection in a transitional stage between FIG. 2 and FIG. 4. It will beseen from FIGS. 3 and 4 that in the region of the arcuate guide pathsection 4, the number of spraying nozzles 206 along the outer curvatureside E of the section is equal to the number of spraying nozzles 6 ateach of the other sides of the section, that is in the exampleillustrated, the unification of the thickness of the marginal crusts onall sides of the cast section behind the region of the bend in the castsection is accomplished by reducing the intensity of the cooling actionon the outer curvature side E of the cast section, which had initiallybeen increased. In conjunction with the unification of the thickness ofthe marginal crusts, the arcuate guide path section 4 serves not onlythe purpose of supporting the cast section S, but also to hold the bentcast section along the scheduled arcuate path, that is against thewarping tendencies of the cast section engendered by the differentialcooling of the sides of the section and by the differential increase inthe thickness of the marginal crusts.

The invention may also be applied in such curved continuous castingplants in which the straight vertical guide path section 3 is followedby a curved guide path section which is not arcuate, that is struckabout a center of curvature, but has a progressively increasingcurvature, e.g. parabolic, hyperbolic or the like, the cast sectiontraversing this guide path section accordingly being bent continually.In this case, it may be sufficient to cool the outer curvature side E ofthe cast section more intensively only in the region of the straightvertical guide path region 3 than the other sides of the cast section,and especially the inner curvature side A. Although the cast sectionrunning from the straight vertical guide path section 3 into the curvedsubsequent guide path section has a marginal crust 108 along the outercurvature of greater thickness than that of the marginal crusts at theother sides of the section, and especially along the inner curvatureside A, the zero-line N N between the tensile and compressive portionsof the crosssection of the cast section retains its position offset fromthe center without change, as reached at the end of the straight guidepath, during the subsequent and progressively increasing bending actionon the cast section S in the curved guide path, that is it does notcreep towards the outer curvature side E of the section, as required foran increase inthe curvature of the section. On the other hand, in thecase of guide path sections for cast sections comprising a progressivelyincreasing curvature, the more intensive cooling action applied on theouter curvature side E of the section in the region of the straightvertical guide path section 3 may also be continued in the region of thesubsequent curved guide path section and may be increased progressivelyin the longitudinal direction of the cast section according to theincreasing curvature of the cast section. Consequently, the differencebetween the thicknesses of the marginal crusts 108 and 208 along theouter and inner curvatures of the cast section also increasesprogressively in the region of the curved guide path section.Accordingly, the zero-line N-N between the tensile and compressiveportions of the cross-section of the cast section travels progressivelytowards the outer curvature side E of the cast section corresponding tothe increase in the curvature of the cast section in the curved guidepath section. In both cases, the difference in the intensity of thecooling action between the more intensive one on the outer curvature ofthe cast section and the less intensive one on the other sides of thecast section may, for unification of the thicknesses of the marginalcrusts of all sides of the cast section, be suddenly or graduallyreduced, cancelled or even reversed, in the terminal region of thecurved guide .path section at an appropriate point prior to the completesolidification of the cast section.

I claim:

1. In a process for the continuous casting of molten metals,

particularly of steel, which process includes the steps of continuouslycasting the metal in a casting mould having a straight vertical castingspace, and passing the metal so cast as a molten core surrounded by aperipheral crust of solidified metal into and through a straightvertical guide path section and a succeeding curved guide path section,the improvement which comprises:

a. commencing while the cat metal is passing through the straightvertical guide path section, cooling while passing through the straightas well as the curved guide path section that side of the cast metalwhich is to form the side of greater radius of curvature moreintensively than the side which is to form the side of lesser radius ofcurvature to provide the side of greater radius of curvature with acrust of greater thickness than that of the side of lesser radius ofcurvature, constraining the difierentially cooled metal casting alongsaid curved guide path section,

. maintaining an intensity of cooling in the region of said straightguide path section approximately 3 to 7 times greater than the intensityof cooling in said curved guide path to so increase the thickness of theperipheral crust in said straight guide path section that the cast metalcan withstand entry into said curved guide path section withoutmechanical damage, and

c. after deflection of the cast metal into said guide path and while thecore is still molten reversing said differential cooling so as to applya cooling of greater intensity to the side of lesser radius of curvatureof the cast metal in the terminal region of the curved guide pathsection whereby the solidified crusts along all sides of said cast metalassume substantially the same thickness and the solidified metal assumesa symmetrical shape at a point prior to the complete solidification ofsaid cast metal.

1. In a process for the continuous casting of molten metals, particularly of steel, which process includes the steps of continuously casting the metal in a casting mould having a straight vertical casting space, and passing the metal so cast as a molten core surrounded by a peripheral crust of solidified metal into and through a straight vertical guide path section and a succeeding curved guide path section, the improvement which comprises: a. commencing while the cast metal is passing through the straight vertical guide path section, cooling while passing through the straight as well as the curved guide path section that side of the cast metal which is to form the side of greater radius of curvature more intensively than the side which is to form the side of lesser radius of curvature to provide the side of greater radius of curvature with a crust of greater thickness than that of the side of lesser radius of curvature, constraining the differentially cooled metal casting along said curved guide path section, b. maintaining an intensity of cooling in the region of said straight guide path section approximately 3 to 7 times greater than the intensity of cooling in said curved guide path to so increase the thickness of the peripheral crust in said straight guide path section that the cast metal can withstand entry into said curved guide path section without mechanical damage, and c. after deflection of the cast metal into said guide path and while the core is still molten reversing said differential cooling so as to apply a cooling of greater intensity to the side of lesser radius of curvature of the cast metal in the terminal region of the curved guide path section whereby the solidified crusts along all sides of said cast metal assume substantially the same thickness and the solidified metal assumes a symmetrical shape at a point prior to the complete solidification of said cast metal. 